Children born to fathers exposed to excessive heat around the time of conception may be at an increased risk of brain cancer, a US study suggests.

Men who used saunas, hot tubs and electric blankets were more likely to father children who went on to develop brain tumours, researchers found.

Experts already know that heat can damage sperm and harm a man's fertility.

The latest work appears in the American Journal of Epidemiology.

Saunas and electric blankets can also reduce the chances of conception so if would-be fathers are concerned then they should avoid these while trying for a baby

Dr Kat Arney of Cancer Research UK

The authors, from the Children's Hospital of Philadelphia, studied the parents of 318 healthy children and the parents of 318 children with two types of brain cancer called medulloblastoma and primitive neuroectodermal tumour.

Brain tumours in children are rare. But these are two of the most common types.

The parents were asked to recall the frequency of exposure to several sources of excessive heat around the time they had conceived their children.

The researchers discovered heat exposure among the men in the three months before the pregnancy appeared to be linked to brain cancer risk among the children, even when other lifestyle factors, such as the father's age and smoking status, were taken into account.

Heat exposure in the mother however did not appear to be important.

Sperm damage

Dr Greta Bunin and colleagues say a likely explanation for their findings is that heat damages the genetic material within sperm, which means cancer-causing genetic mutations are passed on to any offspring.

Normal sperm production requires a body temperature three to four degrees lower than normal body temperature.

But they said the idea that paternal heat exposure before a child's conception could increase the risk of these brain cancers must be considered speculative until more proof of this is found.

"This is worth investigating further, as so little is known about the aetiology [causes] of childhood brain tumours," they said.

Dr Kat Arney, science information officer at Cancer Research UK, said: "Childhood brain cancer is rare and we know little about its causes.

"Primitive neuroectodermal tumour is the third most common kind of brain cancer. There are fewer than 100 cases of PNET every year in the UK.

"Heat sources such as saunas and electric blankets can also reduce the chances of conception so if would-be fathers are concerned then they should avoid these while trying for a baby."